We have seen in the case of the Cerceris, Sphex, and Ammophila how the mother resolves the feeding problem, by placing beforehand in the cell a sufficient quantity of game, and also that of keeping it for weeks perfectly fresh—nay, almost alive, though motionless—in order to secure the safety of the grub which [[234]]feeds on the prey. This marvel is brought about by the most skilful means known to physiology. The poisoned sting is sent into the nerve centres once or oftener, according to the construction of the nervous system, and the victim retains all which we call life, except power of motion.
Let us see if the Bembex practises this deep science of murder. Diptera taken from between the feet of their captor as the latter enters the burrow mostly seem quite dead. They are motionless; only in rare cases are there some slight convulsions of the tarsi—the last vestiges of life soon to be extinct. The same appearance of complete death is found, as a rule, in insects not really killed but paralysed by the skilful stab of a Cerceris or a Sphex. The question as to life or death can, therefore, only be decided by the manner in which the victims keep fresh.
Placed in little paper twists or glass tubes, the Orthoptera of the Sphex, the caterpillars of the Ammophila, the Coleoptera of the Cerceris, preserve flexibility of limb and freshness of colour, and the normal state of their intestines, for weeks and months. They are not corpses, but bodies plunged in a lethargy from which there will be no awakening. The Diptera of the Bembex behave quite otherwise. Eristalis, Syrphus,—in short, all which are brightly coloured,—soon lose their brilliance; the eyes of certain gadflies, magnificently gilded, and with three purple bands, soon grow pale and dim, like the gaze of a dying man. All these Diptera, great and small, placed in paper twists where air circulates, dry up and grow brittle in two or three days, [[235]]while all kept from evaporation in glass tubes, where the air is stagnant, grow mouldy and decay. So they are dead—really dead—when carried to the larva. If some few preserve a little life, a few days, a few hours ends all. Not being clever enough to use its sting, or for some other reason, the assassin kills its victims outright.
Knowing this complete death of the prey at the moment when it is seized, who would not admire the logic of the Bembecid’s manœuvres? How methodical all is, and how one thing brings about another in all which the wary Hymenopteron does! As the food could not be stored without its decaying at the end of two or three days, it cannot be laid in wholesale at the beginning of a phase of life destined to last at least a fortnight, and there must be a hunt and distribution of provisions day by day, in proportion to the larva’s growth. The first ration—that on which the egg is laid—will last longer than the others, and must be small, for the little grub will take several days to eat it, and if too big it would go bad before it was finished. Therefore it will not be a huge gadfly or a corpulent Bombylius, but a small Sphærophoria, or something of that kind, as a tender meal for a still frail larva. Later, and gradually larger, will come the bigger joints.
In the mother’s absence the burrow must be closed to prevent awkward intrusions, but the entrance must be one opened quickly, without serious difficulty, when the Hymenopteron returns loaded with prey, and laid in wait for by audacious parasites. These conditions would be wanting in a tenacious soil, such as that in which the mining Hymenoptera habitually [[236]]establish themselves. The wide-open entrance would each time require long and painful labour, whether to close it with earth or gravel, or to clear it. The domicile, therefore, must be hollowed in earth with a very light surface, in dry, fine sand, yielding at once to the least effort of the mother, and which slips and closes the entrance like floating tapestry, which, pushed back by the hand, allows entrance and then drops back. Such is the sequence of acts, deduced by human reason, and put into practice by the wisdom of the Bembex.
Why does the spoiler kill the prey instead of paralysing it? Is it want of skill with the sting? Is it a difficulty arising from the organisation of the Diptera or from the manœuvres of the chase? I must own, at once, that I have failed to put a Dipteron, without killing it, into that state of complete immobility into which it is so easy to plunge a Buprestis, a Weevil, or a Scarabæus, by injecting a little drop of ammonia, on the point of a needle, into the thoracic ganglia. It is difficult to render your subject motionless; when it no longer moves, actual death has occurred, as is proved by its speedy decay or desiccation. But I have too much confidence in the resources of instinct,—I have seen the ingenious solution of too many problems,—to believe that a difficulty, though insurmountable for the experimenter, can baffle an insect; therefore, without casting doubt on the Bembex’s capacity for murder, I should be inclined to seek other motives.
Perhaps the Dipteron, so thinly cuirassed, of so little substance,—so lean, in short,—could not, when [[237]]paralysed by a sting, resist evaporation, and would dry up in two or three weeks. Consider the slender Sphærophoria—the larva’s first mouthful. What is there in this body to evaporate? An atom—a mere nothing. The body is a thin strip—its two walls touch. Could such prey form a basis for preserved food when a few hours would evaporate its juices, unrenewed by nutrition? To say the least, it is doubtful.
Let us proceed to consider the manner of hunting, by way of throwing a final light on the subject. In prey withdrawn from the clasp of a Bembex, one may not infrequently observe indications of a capture made in haste, as best might be, in the chances of a wild struggle. Sometimes the Dipteron has its head turned backward, as if its neck had been twisted, its wings are crumpled, and its hairs, if it have any, are ruffled. I have seen one with the body ripped open by a bite from the mandibles, and legs lost in the battle. Usually, however, the prey is intact.
No matter. Considering that the game has wings prompt in flight, the capture must be made with a suddenness which it seems to me hardly allows of obtaining paralysis without death. A Cerceris with its heavy weevil, a Sphex engaged with a corpulent grasshopper or a paunched ephippiger, an Ammophila holding its caterpillar by the nape of its neck, have all three the advantage over a prey too slow to avoid attack. They may take their time, choose at leisure the exact spot where the sting shall penetrate, and, in short, can act with the precision of a physiologist who uses his scalpel on a patient laid upon the [[238]]operating table; but for the Bembex it is another matter. At the least alarm the prey is off, and its power of wing defies that of the pursuer. The Hymenopteron must pounce on its prey, without measuring its attack or calculating its blow, like a hawk hunting over the fallows. Mandibles, claws, sting—all weapons—must be used at the same moment in the hot battle, to end as fast as possible a struggle in which the least indecision would give the prey time to escape. If these conjectures agree with facts, the Bembex can only secure a dead body, or, at all events, a prey wounded to death.
Well, my calculations are right. The Bembex attacks with an energy which would do honour to a bird of prey. To surprise one on the chase is no easy matter, and it would be useless to lay in a stock of patience and watch near the burrow, for the insect flies to a distance, and it is impossible to follow its rapid evolutions, and doubtless its manœuvres would be still unknown to me but for the help of an article from which I should assuredly never have expected a like service—namely, the umbrella which served me as a tent amid the sands of Issarts.